In recent years, high sensitization has been required for magnetic recording because it improves image and sound qualities and permits high density recording thereof. This high sensitization can be realized by improving a deck for reproduction and recording of magnetic records, a magnetic recording system of a magnetic recording medium, and also by improving the magnetic recording medium itself and so forth.
The high sensitization through improvement of the magnetic recording medium can be attained by increasing the signal part of the signal/noise ratio or by decreasing the noise part thereof. Increasing the signal part of the signal/noise ratio can be accomplished by decreasing the size of ferromagnetic particles in the magnetic layer, or by controlling the anisotropy of shape and the arrangement of a monomagnetic zone in the ferromagnetic powder so that the residual magnetic flux density and the coercive force of the magnetic recording medium are increased.
In decreasing the noise part, various factors are involved, including the smoothness of the magnetic layer and the charging characteristics of the magnetic recording medium. In order to control the charging characteristics and to maintain running durability, it has been proposed that a backing layer is provided. Details of such backing layers are described in U.S. Pat. No. 4,135,031 and Japanese Patent Publication No. 3927/75, Japanese patent application (OPI) Nos. 111828/82 and 96505/77 (the term "OPI" as used herein means a "published unexamined Japanese patent application").
High sensitive magnetic recording media having good running durability are quite difficult to produce even by employing the methods described in the above references. The major reasons are as follows.
(1) If the surface of the magnetic recording medium is smoothed, the spacing loss between the magnetic recording medium and the head for reproduction and recording of magnetic records is decreased, but its running durability is reduced and, furthermore, its coefficient of friction is increased, causing problems such as the peeling off of the magnetic layer.
(2) If the surface of the backing layer is roughened to increase its running durability, the roughness in the surface of the backing layer is printed through the magnetic layer. This leads to a reduction in the RF output and the signal/noise ratio of the magnetic layer.
(3) Furthermore, even at the initial running stage where the peeling off of the magnetic layer or the back layer does not occur, the number of drop outs is increased and the signal/noise ratio is decreased.
(4) Calendering treatment is known as one of the techniques for smoothing the surface of the magnetic layer for the high sensitization thereof (see, for example, Japanese Patent Publication No. 10244/74). However, if a degree of smoothness of the magnetic layer achieves the required high sensitization, the backing layer is peeled apart and, therefore, the desired magnetic recording medium is difficult to produce.